1. Field of the Invention
The present invention relates to back lights and liquid crystal displays, and more particularly to a back light illuminating a liquid crystal panel with light from a back surface and a liquid crystal display provided with the same.
2. Description of the Background Art
Recently, the liquid crystal display has a wide range of applications because of its low power consumption, light weight and small thickness. The liquid crystal display is used, for example, as a display of a household video monitor, industrial machine monitor or personal digital assistant to be fit to use. In particular, the personal digital assistant is increasingly becoming low in power consumption, light in weight, and small in picture-frame and thickness.
FIG. 13 is a cross sectional view showing a conventional structure of a back light illuminating a liquid crystal display with light from a back surface. As shown in FIG. 13, the back light includes a reflection sheet 3 and a frame 8 below a light guide plate 22 uniformly transmitting light. A tubular light source 1 is arranged near the end surface (edge) of light guide plate 22. A reflector 2 is arranged to partially surround tubular light source 1.
Provided above light guide plate 22 are a sheet having a light collecting effect, two sheets 6 having a light diffusing effect (each provided with grooves both in longitudinal and lateral directions), and another sheet 5 having the light collecting effect. A liquid crystal panel 7 is arranged thereabove. Light guide plate 22 is formed of transparent acrylic resin, and its surfaces are all smooth and planar.
It is noted that, the phrases "above light guide plate 22" and "below light guide plate 22" are respectively used to indicate directions toward liquid crystal panel 7 and frame 8 from light guide plate 22.
As the back light has four sheets in total, the overall thickness of the liquid crystal display is large. Further, provision of two sheets 6 having the light diffusing effect makes the liquid crystal display more expensive.
Then, a back light as shown in FIG. 14 has been developed to reduce cost as well as thickness and weight of the liquid crystal display. In the back light, two sheets are eliminated to reduce the overall thickness of the liquid crystal display, so that reduction in thickness and weight is achieved.
FIG. 14 is a cross sectional view showing the structure of the conventional back light illuminating the liquid crystal display from a back surface. FIG. 15A is a cross sectional view showing a structure of frame 8 taken along the line 15A--15A in FIG. 15B.
Referring to FIG. 14, the back light is provided with a reflection sheet 3 and a frame 8 below a light guide plate 4 having a light collecting effect. A tubular light source 1 is arranged near the end surface (edge) of light guide plate 4. A reflector 2 is arranged to partially surround tubular light source 1. Provided above light guide plate 4 are a sheet 5 having the light collecting effect and a sheet 6 having a light diffusing effect. A liquid crystal panel 7 is arranged thereabove.
Instead of having two sheets, light guide plate 4 has at the bottom a lens like grooved surface 4a having the light collecting effect. Further, as shown in FIGS. 15A and 15B, an opening is formed in frame 8 supporting the light guide plate for reduction in weight and thickness and in consideration of a positional relationship with respect to an electric circuit or mechanical part.
In the case of light guide plate 4 having lens like grooved surface 4a at the bottom as described above, lens like grooved surface 4a may be brought into contact with frame 8 or a part supporting light guide plate 4. In such case, lens like grooved surface 4a is damaged, thereby causing irregular reflection. In some cases, reflection sheet 3 may be caught by and fixed to lens like grooved surface 4a. As a result, display by the liquid crystal display becomes white or bright (the phenomenon is called a luminous dot phenomenon).
Furthermore, as shown in FIG. 15A, sharp edge 16 of frame often causes damage to lens like grooved surface 4a, possibly resulting in the luminous dot phenomenon. In particular, if edge 16 is within a liquid crystal display area, sharp edge 16 would more often cause damage to lens like grooved surface 4a, possibly resulting in the luminous dot phenomenon.
In addition, lens like grooved surface 4a of light guide plate 4 may be damaged by a protrusion 20 by a pin or a protrusion by a gate pin, which are caused during manufacture.